1. Field of the Invention
The present invention relates to particulate spacers for use as spacers in liquid crystal display cells, spacers in electrochromic display devices, or other spacers for maintaining a constant distance between films, sheets or blocks. The present invention also relates to liquid crystal display cells employing such particulate spacers.
2. Description of Prior Art
Liquid crystal display cells have a pair of transparent substrates with electrodes, a spacer which maintains the two substrates at a constant distance, a liquid crystal composition placed between the two substrates and a sealing material for sealing the periphery of the gap space between the two substrates. Since a fixed gap must be maintained between the pair of substrates in the liquid crystal display cell, organic or inorganic solid particles of nearly constant diameter are placed between the two substrates at prescribed intervals as spacers. Examples of this type of particles are disclosed in Japanese Laid-Open Patent Publications No. 57-189117, 59-24829, 62-296122 and 63-200126.
However, in conventional liquid crystal display cells, these particles are not fixed to the substrates, and consequently the following shortcomings arise.
(1) Air blown onto the substrates in the process of assembling the liquid crystal display cell may cause scattering of particulate spacers, resulting in loss of the spacers.
(2) The particulate spacers may be shifted on the surface of the substrates in the process of injecting liquid crystal composition into the liquid crystal display cell.
(3) The particulate spacers may be displaced by the electrical or hydrodynamic forces arising while the liquid crystal display cell is in operation.
Elimination of these shortcomings requires that the particulate spacers be fixed to the substrates.
Furthermore, because the particulate spacers used in the conventional liquid crystal display cells cannot shield light, a high-contrast display cannot be obtained in the liquid crystal display cells. Therefore, colored particulate spacers that can shield light have been examined. Colored particles may be used as such particulate spacers, as desired. The following methods have formerly been proposed for the purpose of preparing colored particles.
1) A pigment is dispersed in a monomer, and polymerization is carried out, resulting in colored particles.
2) An oil-soluble dye is dispersed or dissolved in a monomer and polymerization is carried out, resulting in colored particles.
3) A reactive dye, formed by introducing polymerizable unsaturated groups into certain dye molecules, is subjected to copolymerization.
4) Colorless particles are produced first, and these particles are then colored with dyes.
However, as regards the aforementioned method 1, the uniform dispersion of the pigment in the monomer is difficult, and consequently uncolored transparent particles are also formed.
As regards the aforementioned method 2, the solvent tolerance of the colored particles so obtained is poor. Moreover, the dye may be decolorized by the polymerization catalyst. Furthermore, since dyes possess polymerization inhibiting properties, the monomer in such a mixture cannot be completely polymerized in some cases.
As regards the aforementioned method 3, since a reactive dye is used as one of the initial raw materials, the choice of initial raw materials is correspondingly restricted. Moreover, the resulting polymer does not possess a high degree of polymerization.
As regards the aforementioned method 4, particles dyed to a deep coloration are not obtained. Moreover, the dye easily dissociates from the resulting colored particles if these particles are in contact with organic solvents.